The present invention relates to a composite second-order intermodulation (to be referred to as a distortion factor hereinafter) reduction technique in an optical communication transmission line (including an optical repeating amplifier ).
Optical fiber communications have been used in the field of public communications, traffic systems, and private communication networks of power systems, and are expected to become important techniques in future societies highly oriented with information. The optical fiber communications have advantages in low losses, wide bands, lightweight, non-induction, and energy savings over communications using metallic cables. Low-cost, high-reliability communication systems, and construction and maintenance of these communication systems can be facilitated. Optical fibers are classified into a single mode fiber and a multimode fiber. The single mode fiber is better in loss and band characteristics than the multimode fiber and is expected to be popular.
An optical fiber communication system employs a direct intensity modulation scheme. This transmission scheme includes a digital transmission scheme and an analog transmission scheme. A digital transmission scheme is generally used in a communication network. In an analog transmission scheme, a light source is directly intensity-modulated or frequency modulated using an frequency division multiplexing input signal. on a receiving side, the light is converted into an electrical signal, and the electrical signal is amplified. The amplified signal is then filtered through a band-pass filter, thereby easily performing signal transmission. The analog transmission scheme is assumed to be suitable in private branch transmission, subscriber trunk transmission, and video transmission.
The present inventor performed analog transmission for optical amplification using an Er-doped optical fiber. In this case, the present inventor found that the distortion factor was increased using an optical fiber having a large polarization mode dispersion (PMD). In the analog transmission scheme, mode distribution noise is predominant in the multimode fiber, while reflected noise is predominant in the single mode fiber (Hikari Tsushin Handbook). The increase in distortion factor was not based on either noise, but was based on the polarization mode dispersion according to the finding of the present inventor.
Of all the transmission characteristics of optical fibers, band characteristics are determined by the dispersion (difference in transmission speed). If the core of a single mode fiber is truly circular, the transmission mode is degenerated, and band characteristics determined by wavelength dispersion are obtained. However, in practice, the core cannot be truly circular, degeneracy is canceled, and the transmission speed changes on a polarization plane (C. D. Poole, Optics Letters, Vol. 14, No. 10, pp. 523-525), thereby increasing the distortion factor. To solve the problem on the increase in distortion factor is to improve the transmission quality in the analog transmission scheme and to improve the band characteristics, thereby improving the digital transmission scheme. Along with highly advanced future optical communication techniques, it is possible to further improve communication systems.